Printed circuit boards (PCBS) are commonly used to provide a mechanical structure on which discrete electronic components, such as integrated circuits, resistors, capacitors and the like, are mounted and interconnected. The interconnections in a PCB are provided by layers of patterned traces of metal, typically copper, on and in the PCB. To meet more compact requirements, multi-layered PCBS with multiple layers of patterned metal traces are used. Such PCBS comprise alternate layers of insulation and patterned metal traces, with interconnections between the patterned metal traces through the layers of insulation by vertical vias or plated through holes. Typically, in a four layer PCB i.e. one having four patterned layers of metal traces, the two inner most layers provide a common electrical power plane and a common electrical ground plane, respectively, for all the circuitry on the PCB.
The operation of circuitry on a multi-layered PCB, especially where there signals at high frequencies of around 50 megahertz or more, causes voltage fluctuations between the power and ground planes. Such fluctuations, often referred to as electromagnetic interference (EMI) or noise, is undesirable. The common ground and common power planes in a multi-layered PCB have a capacitance value which helps to reduce EMI, however for most applications a larger capacitance is required to further reduce the EMI to meet EMI standards set by state regulatory bodies.
A method of controlling EMI is to decouple the power and ground planes by inserting discrete capacitors at various locations on a PCB, particularly at locations where the power and ground planes couple to power input terminals of an integrated circuit mounted on the PCB. A disadvantage of this method is that discrete capacitors consume scarce real estate on the PCB, and in addition, at high frequencies, discrete capacitors become less efficient in providing effective decoupling.
Another relatively new method of controlling EMI is disclosed in U.S. Pat. No. 5,079,069 assigned to Zycon Corporation, which provides a capacitive PCB that includes a capacitor laminate having a high capacitance value between the power and ground planes. The capacitor laminate extends through out the PCB, including locations with signal traces. The high capacitance of the capacitor laminate provides effective decoupling of EMI, however, a disadvantage of this method is that the high capacitance value of the capacitor laminate reduces the trace impedance of the signal traces.
This is because the capacitance of the capacitor laminate is inversely proportional to the thickness of the laminate, while signal trace impedance is directly proportional to the thickness of the laminate. Consequently, the high capacitance of the capacitor laminate has a relatively small thickness to provide improved decoupling, but at the same time the small thickness of the capacitor laminate reduces the impedance of the signal traces to below desired impedance values.
The signal trace impedance is inversely proportional to the width of the trace, hence a common method of increasing the signal trace impedance to desired values when using the capacitor laminate is to reduce the width of the signal trace. Therefore, while using the capacitor laminate improves decoupling in a PCB, there is presently a consequent need to reduce the width of signal traces on the PCB to ensure the signal traces have the desired trace impedance values. The resultant difficulty then when using the capacitor laminate in a PCB is fabricating the relatively narrow traces that are required.